The capacitor serves as a short time current source and sink for the switching semiconductors.

To provide effective protection against short-duration excess currents, the build-up rate of the current is limited by a choke (9) in the current sensor (8).

The connection points may in one case be a number of sensors which in the form of time division multiplex represent the separate sensors by current pulses of short duration on the main current circuit.

This is sufficient to cope with a short power outage, but not enough to cover one lasting three hours.